1. Field of the Invention
The present invention relates to a vacuum pump, and more particularly to a vacuum pump capable of effective evacuation in pressure ranges from an atmospheric pressure to a high vacuum.
2. Description of the Related Art
FIG. 31 is a schematic diagram of a semiconductor manufacturing apparatus with a conventional vacuum pump.
As shown in FIG. 31, a semiconductor manufacturing apparatus 101 has a plurality of process chambers 102, a transfer chamber 103 and a cassette chamber 104. A wafer (substrate) to be processed is placed in the cassette chamber 104, transferred by way of the transfer chamber 103 to the process chamber 102, where it is subject to a predetermined process (such as PVD, CVD, and etching). A plurality of process chambers 102 are commonly provided in order to perform a plurality of processes or to increase the number of wafers to be processed in the single semiconductor manufacturing apparatus 101.
It is necessary to create a high vacuum state in the process chamber 102 before processing, and to exhaust a process gas continuously from the process chamber 102 during processing. To this end, a turbo molecular pump 105 is widely used as a vacuum pump for vacuum evacuation of the process chamber 102. While the turbo molecular pump 105 is operable in moderate and high vacuum ranges at the order of 101 Pa or below, it cannot operate independently under atmospheric pressure. Therefore, a backing pump 106 for preliminary evacuation is connected to the exhaust port of the turbo molecular pump 105 through piping 107. The backing pump 106 is configured to evacuate a gas at a pressure from atmospheric pressure to the order of 101 Pa.
A semiconductor manufacturing apparatus with the above configuration requires two types of vacuum pumps, namely the turbo molecular pump 105 and the backing pump 106, for each process chamber 102, as pumps for exhausting a gas therefrom. Therefore, there has been a problem of an increased space for installation, an increased number of components, a high cost, and so on. In recent years, the volume of gas used in semiconductor processing has tended to increase, which in turn has caused the vacuum pumps to be upsized, and the piping 107 to be upsized in diameter as well. Thus, the above problem becomes conspicuous.
There is mainly used a positive displacement pump, such as a roots pump, a screw pump, and an oil rotary pump, as the backing pump 106. This type of pump is configured that a rotor rotating at a relatively low speed reduces the volume of an exhaust flow passage in an exhaust chamber (casing) gradually to transfer a gas. Therefore, in order to increase the volume of gas transferred, the volume and the mass of the rotor need be increased, which unavoidably accompanies upsizing of the backing pump.
As measures for the above problem, there is a method in which the rotor is rotated at a high speed, in order to increase the volume of gas transferred without upsizing the backing pump. However, a roots pump and a screw pump have two main shafts, to each of which a rotor is fixed, and require a mechanism to constrain the rotation phases of the two main shafts (such as timing gear), and thus are not suitable for high-speed rotation. An oil rotary pump has an asymmetric rotor with respect to the rotation axis, and thus is not suitable for high-speed rotation, either. Thus, it is extremely difficult to downsize a backing pump with increasing the volume of gas transferred by causing the backing pump to rotate at a high speed.
In the above backing pumps, an oil such as lubricant is used in a bearing or a sealed portion, which hinders creation of a completely oil-free vacuum. This causes the quality and yield of products manufactured by the semiconductor manufacturing apparatus to be reduced.
In view of the above problems, Japanese Patent, JP-B-03-007039 discloses a vacuum pump capable of efficient vacuum evacuation from an atmospheric pressure to a high vacuum solely. The vacuum pump disclosed in the above Japanese Patent includes a centrifugal compression pump step having a plurality of impellers, and a circumferential flow compression pump step. However, the impellers are all attached to a tip of the main shaft, and therefore the rotor is a cantilever rotor with the tip of a large mass distribution. This deteriorates the vibration characteristics of the rotor and makes it difficult for the rotor to rotate at a high speed, causing a problem that the pump cannot be downsized. Furthermore, since a lubricant or the like is used in the bearing section, it was not possible to create a completely oil-free vacuum.
A vacuum pump disclosed in Japanese Patent, JP-B-07-086357 aims to adapt the rotor for high-speed rotation with the use of a magnetic bearing. However, since the magnetic bearing is located in the vicinity of an intake port, the magnetic bearing produces a resistance to exhaust gas, causing a problem that the exhaust performance of the vacuum pump was impaired. In particular, when the pressure on the intake port side is in the molecular flow range, the exhaust conductance reduces significantly, that is, the resistance to exhaust gas increases, causing a problem that the effective exhaust rate reduces conspicuously. Since a liquid coolant is introduced into a space connected to the exhaust flow passage of the pump, there also was a problem that the liquid coolant may contaminate the vacuum environment.
In the meantime, a turbo vacuum pump including a centrifugal drag pump element is occasionally used for vacuum evacuation of a process chamber of a semiconductor manufacturing apparatus. This type of turbo vacuum pump is described with reference to the drawings. FIG. 32 is a sectional view of a conventional turbo vacuum pump. FIG. 33(a) is a plan view of a centrifugal drag vane shown in FIG. 32, and FIG. 33(b) is a sectional view of the centrifugal drag vane shown in FIG. 32. FIG. 34(a) is a plan view of a fixed vane shown in FIG. 32, and FIG. 34(b) is a sectional view of the fixed vane shown in FIG. 32.
As shown in FIG. 32, the turbo vacuum pump comprises centrifugal drag vanes 133 constituting plural stages, a plurality of fixed vanes 134 disposed to face each of the stages of the centrifugal drag vanes 133, and a casing 108 having an intake port 111 and an exhaust port 131. The centrifugal drag vanes 133 are fixed to a main shaft 195, and are driven to rotate by a motor 123 through the main shaft 195. The main shaft 195 is supported in a non-contact manner by an upper radial magnetic bearing 122, a lower radial magnetic bearing 144, and an axial magnetic bearing 143. An upper touchdown bearing 126 and a lower touchdown bearing 147 are disposed above the upper radial magnetic bearing 122 and below the lower radial magnetic bearing 144, respectively.
As shown in FIG. 33(a) and FIG. 33(b), each of the centrifugal drag vanes 133 has a plurality of spiral blades 135 extending rearward with respect to the rotation direction, and a disk-shaped base 109 to which the spiral blades 135 are fixed. On the other hand, as shown in FIG. 34(a) and FIG. 34(b), the fixed vane 134 has a plurality of spiral guides 166 extending rearward with respect to the rotation direction of the centrifugal drag vanes 133, and an annular plane portion 167 to which the spiral guides 166 are fixed. The arrows G shown in FIG. 33(a) and FIG. 34(a) indicate the flow of a gas.
When the centrifugal drag vanes 133 are rotated in the direction of the arrow Q, a gas is drawn into the casing 108 from the intake port 111, and is compressed as it is transferred toward the radially outer side through action of a centrifugal force. The gas having been transferred to the radially outer side, then flows into a space defined by the spiral guides 166, the plane portion 167, and the backside of the base 109, and the gas is compressed as it is transferred toward the radially inner side through drag action due to viscosity of the gas. In this manner, the gas is transferred at each stage to be compressed to a desired pressure, and discharged through the exhaust port 131.
In the conventional turbo vacuum pump, however, since the number of stages of the vanes was simply increased to improve the exhaust performance, the exhaust efficiency remained low. Therefore, a problem was raised that the exhaust rate reduced and that the compression ratio remained low, which as a result caused upsizing of the entire turbo vacuum pump and an increase in manufacturing costs.
The present invention has been made in view of the foregoing, and it is therefore an object of the present invention to provide a vacuum pump capable of evacuating in pressure ranges from an atmospheric pressure to a high vacuum, capable of rotating at a high speed to be downsized and improved in pumping performance, and capable of producing a completely oil-free vacuum.